In the prior art, a fiber Bragg's grating (referred to as FBG hereinafter) is formed by an optical fiber to serve an optical fiber filtration function. The FBG can reflect the wavelength as reception of light waves according to a default Bragg's feedback wavelength λB engendered by a default grating period Λ of the fiber grating, serving as an accurate filtration device for optical fiber communication. In an FBG by using a feedback effect generated Bragg diffraction, a predetermined wavelength satisfying a Bragg condition, referred to as a feedback Bragg wavelength λB, is reflected in a direction reverse to the incident direction back to a scanning apparatus that emits light waves for further analysis, so as to measure whether a received wavelength is increased or decreased; or the reflected wavelength is split to enter a communication receiving device, so as to detect the modulated carrier signal content in the preset received wavelength. The feedback Bragg wavelength 4 is represented by the following equation:λB=2nΛ  (1)
In equation (1), Λ is the period of the FBG and n is an effective refractive index of the optical fiber. The wavelength value of the feedback Bragg wavelength λB can change as the refractive index n of a core material of the optical fiber increases or decreases due to a change in temperature or the interval of Λ is changed due to force received by the fiber grating.
When temperature does not change and the FBG is used as a strain measurement function, a variance in the original interval of Λ which is caused by the strain generated in the fiber grating by an external force received is ΔΛ, which is substituted into equation (1) to obtainΔλB=2nΔΛ  (2)
According to the definition of strain ε, the gauge length of the force receiving object is set to 1, and Δ1 is the length change due to the force receivedε=Δ1/1=ΔΛ/Λ  (3)It is then obtained that:Δ1=(ΔΛ/Λ)1=((ΔλB/2n)/(λB/2n))1,Therefore:ε=Δ1/1=ΔλB/λB  (4)
What is mentioned above is the calculation formula which is used to measure a strain value of a structure to which the FBG attached usually with the preset wavelength variance ratio, i.e. ΔλB/λB, under the assumption of the refractive index n being a fixed value when temperature does not change.
However, when temperature changes, the measurement of the ratio of a wavelength variance ΔλB to a variance in the original preset Bragg feedback wavelength λB is usually represented by the following equation:Δλβ/λβ=(1−Pe)Δε+(αf+ξ)ΔT  (5)
Pe is an effective photo-elastic effect value, αf is a thermal expansion coefficient, and ξ is a thermal-optic coefficient of a fused silica fiber. Δλβ is a variance in the feedback wavelength of the FBG caused by temperature, Δε is a strain variance from the axial direction of the grating caused by temperature change, and ΔT is a temperature variation.
In fact, as the density of glass molecules in a light guide core of the optical fiber changes due to temperature change, the refractive index n changes; and as a result, even though the FBG does not receive any force, the original preset Bragg feedback wavelength λB will still shift. For the requirement on the correctness of point-to-point fixed wavelength communication in optical fiber communication, this often results in the consequence of information missing, so it must be improved. The circuit cost for maintaining constant temperature in any communication use environment to keep the refractive index in the FBG unchanged is too high, so the principle of the natural physical technology and a method with the lowest cost must be utilized to compensate the shift of a fixed communication wavelength λ caused by temperature.
The temperature compensation technology for the wavelength shift of the Bragg feedback wavelength λB caused by temperature change in the prior art has been achieved by structures, such as devices (shown in FIG. 3 and FIG. 4 in the content) disclosed in “Incorporated Bragg Filter Temperature Compensated Optical Waveguide Device” of U.S. Pat. No. 5,042,898 by Morey, et al. as well as devices (shown in FIGS. 6-8 in the content), and conventional technologies (shown in FIGS. 3-5 in the content), disclosed in “Thermal compensated compact Bragg gating filter” of U.S. Pat. No. 6,493,486 by Chen in Finisar Corporation. Most of the above-mentioned various devices or structures adopt two types of metal materials with different expansion coefficients to form various structures with different geometrical shapes, such as a double-metal strip, C-shape clamp or axial rod-shaped structure; and when a cold and hot temperature change causes the physical length difference of such a structure, the FBG in the structure is forced to generate a variance ΔΛ in the grating period due to effects such as stretching or contraction under axial force, stretching or contraction caused by bending under lateral force or stretching or contraction under torsional force. he purpose of deliberately applying the variance ΔΛ on the structure is to reversely compensate an increase or decrease in the physical length of the FBG caused by cold or heat, causing an effect of eliminating the variance ΔΛ in the original preset Bragg grating period.
The specification describes the device in U.S. Pat. No. 5,042,898 by Morey with FIG. 1 and the device in U.S. Pat. No. 6,493,486 by Chen with FIG. 2. FIG. 1 is a sectional view of the device in prior U.S. Pat. No. 5,042,898; in the figure, 20 is an optical fiber filter, 10 is an optical fiber which gets into and out of the fiber filter, 13 is a Bragg grating section, 17 is a section of the optical fiber filter 10 which gets into the optical fiber filter, 21 (28) is a first temperature compensated component, 22 (29) is a second temperature compensated component, 23 is a concave in the first temperature compensated component 21, 24 is a projection of the second temperature compensated component 22, 25 is a bridge for two sections 17 and 13 of the optical fiber connected between an entering end 26 of the optical fiber 10 and a fixing point 27 on the projection 24, 30 is an optical fiber-pretensioning component, and 31 is a restorable for applying pretensioning force. FIG. 2 in the description is a sectional view of prior U.S. Pat. No. 6,493,486, i.e. the device of the sectional view in FIG. 6 in U.S. Pat. No. 6,493,486 by Chen; in the figure, 80 is a temperature compensated Bragg grating filter, 82 is an optical fiber which gets into and out of the fiber filter, 90 is a plastic-jacketing of optical fiber, 84 is an axial optical fiber core, 86 is an optical fiber cladding, 88 is an fiber Bragg's grating section, 92 is an optical fiber component which is twisted with the optical fiber 82, 104 is a twist adjustment component, 106 is a temperature compensation component paired with the twist adjustment component 104, 96 is a fixed connection point for the twist adjustment component 104, 98 is a fixed connection point for adjusting the twist pitch along with the fixed connection point 96 in the temperature compensation component 106, and 94 is a fixed bonding point for the twist adjustment component 104 and the temperature-compensated component 106 paired therewith.
In fact, there is a proportional dependency relationship between the refractive index and the internal stress of the optical fiber. Therefore, when temperature rises, if stress or torsion is released, the refractive index can be decreased to compensate the shift of the wavelength of the grating filter caused by temperature. As shown in FIG. 1, the springs 31 in the optical fiber-pretensioning components 30 restorably apply force to pretension the fiber grating, i so that non-axial stress is generated in the fiber grating. When temperature increases to make the projection 24 made of a high-expansion coefficient material of the second temperature-compensated component 22 extend to release the stress of the FBG 13 of the optical fiber, i.e. release stress, the refractive index can be decreased to compensate the shift of the wavelength of the grating filter caused by temperature. As shown in FIG. 2, the springs 31 in the optical fiber-pretensioning components 30 restorably apply force to pretension the fiber grating, so that non-axial stress is generated in the fiber grating. When temperature increases to make the fiber grating section 88 twisted by the high-expansion coefficient material 96 of the twist adjustment component 104 relieved so that the original torsion for twisting the FBG of the optical fiber is decreased, i.e., torsion is released, the refractive index can be decreased to compensate the shift of the wavelength of the grating filter caused by temperature. The abovementioned technical structures of the prior inventions also makes full use of equation (5) to keep Δλβ unchanged, i.e., ΔλB=0 leads to the left expression Δλβ/λβ=0; and consequently, even if temperature changes, the axial strain variance Δε of the grating caused by temperature change ΔT is adjusted in inverse proportion. When temperature increases, stress or torsion is released; and when temperature decreases, stress or torsion is increased, so that Δλβ is equal to 0, achieving the objective of temperature compensation. Although the abovementioned mechanical structures methods for adjusting the FBG fiber grating in the prior art have made progress in gradually reducing size and reducing cost, when a preset central feedback wavelength of the FBG really needs to be chosen, besides the combination of components made of materials with different expansion coefficients, pretensioning force-applying component devices or components for twisting the FBG must also be produced, and as a result, the temperature compensated structure is enlarged. In particular, such a structure can be regarded as a very large structure for a thin optical fiber (for example, having an external diameter of 250 μm). Especially, distribution frames with very dense optical fibers in a telecommunication room cause space waste, so the size must be reduced in order to satisfy the demand on distribution frame spaces when “Fiber To The Home” is truly fully realized.